Along with the improved performance and improved functions of automobiles, the performance demanded from tires has become higher year and year. As a part thereof, while maintaining the grip on wet road surfaces, that is, the wet grip, development of tires contributing to lower fuel consumption is being strongly demanded. The reinforcing filler, which had been used for tire treads in the past, was carbon black, but recently, due to the above demand, silica, which has excellent low hysteresis loss and wet skid property compared with carbon black, has begun to be used as a reinforcing filler for tire treads (e.g., see U.S. Pat. No. 5,227,425). However, a silica-based filler has hydrophilic silanol groups on the surface thereof and, therefore, is inferior to carbon black in the affinity thereof to rubber molecules. For this reason, while a silica-based filler is superior in the low hysteresis loss and wet skid property, there was the problem that the reinforcibility thereof and the abrasion resistance thereof did not reach those of carbon black. Therefore, to make the reinforcibility of silica-based fillers the same extent as carbon black, a silane coupling agent chemically bonding the rubber molecules with the surfaces of the silica particles so as to be able to increase the reinforcibility has been jointly used (for example, see U.S. Pat. No. 3,978,103). As a typical silane coupling agent, bis(3-triethoxysilylpropyl) tetrasulfide may be mentioned.
However, when mixing a silane coupling agent into a silica-containing rubber composition, there were the problems that if the coupling reaction between the silica and the silane coupling agent was insufficient, excellent dispersability of the silica could not be obtained while if the coupling reaction was excessive, rubber scorching was invited and the quality fell. For this reason, with the conventional kneading method, during the kneading work, a thermocouple etc. was used to measure the rubber temperature at all times and the kneading operation was performed within an empirically set time while maintaining the rubber temperature in a constant range, but there was the problem that the amount of reaction of the silica and silane coupling agent was not necessarily constant with each batch and balancing the mixability of the silica formulation and the desired rubber properties was extremely difficult.
In this regard, for silica-containing rubber compositions, in the past, diphenyl guanidine (DPG) has been made much use of as a vulcanization accelerator. However, in recent years, the detrimental effect on rubber/steel cord bonding have been feared and there has been a movement toward reducing the amount of DPG used. However, in large silica-content rubber compositions, there was the problem that if the amount of use of DPG was slashed, the vulcanization speed dropped and the dispersion of the silica in the rubber deteriorated.
Japanese Patent Publication (A) No. 2005-112921 discloses the compounding, into a diene-based rubber, of a secondary amine compound having a piperidine skeleton, i.e., 2,2,6,6-tetramethyl piperidine or derivatives thereof so as to be able to obtain a rubber composition having a high grip performance, but this publication does not describe the compounding of said compound, together with silica, and therefore, there is no description therein relating to the dispersibility or workability of silica.
Japanese Patent Publication No. 2006-509851 A describes the compounding, into a diene elastomer polymer, of an organic quaternary ammonium salt together with silica or another additive so as to obtain a vulcanized product exhibiting excellent mechanical properties, while maintaining an allowable vulcanization speed even when a secondary vulcanization accelerator is not added. This publication describes the use of 1,4-diazabicyclo[2,2,2]octane for preparation of an organic quaternary ammonium salt (see Examples 1 and 2), but does not describe, at all, the compounding of 1,5-diazabicyclo[4,3,0]nonene-5 (DBN) into a rubber composition.
In this regard, as explained above, had reduction of the amount of use of DPG, which is feared to have a detrimental effect on rubber/metal bonding, as one of their main tasks, but in compounds of formulations containing large amounts of silica, slashing the DPG results in a drop in the vulcanization speed and a deterioration in the dispersion of the silica. In consideration of these problems, the present inventor found, as disclosed in the Japanese Patent Application No. 2007-041005 (i.e., International Publication No. WO 2008/102513), that by compounding a compound having a piperidine skeleton, i.e., Quinuclidine or Quinuclidinol, in a compound of a formulation containing a large amount of silica, it is possible to improve the vulcanization speed and silica dispersion, reinforcibility and viscoelastic properties and the decrease in the compounding amount of DPG. However, since the molecular weights of these compounds are small, the speed of migration to adjacent parts of a pneumatic tire is fast and the detrimental effect on the rubber/metal bondability was insufficiently suppressed.